Touch up [[clang::require_constant_initialization]] docs
Touch up [[clang::require_constant_initialization]] docs

* Fix an egregious comma usage.
* Remove the `static` keyword in the example since the variables should have
  external linkage.
* Use C++11 attributes in the example.


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@281712 91177308-0d34-0410-b5e6-96231b3b80d8

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